Oil production by steam drive



3,425,492 OIL PRODUCTION BY STEAM DRIVE Ralph E. Gilchrist,Bartlesville, kla., assignor to Phillips Petroleum Company, acorporation of Delaware No Drawing. Filed Jan. 10, 1966, Ser. No.519,422 US. Cl. 166-272 7 Claims lnt. Cl. E21b 43/24, 43/20 ABSTRACT OFTHE DISCLOSURE Oil is produced from a permeable oil stratum penetratedby an injection well and at least one olfset production well by firstinjecting steam into the stratum thru the injection well to form a steambubble around this well, injecting solely a gas, other than steam, thruthe injection welldirectly behind the steam to drive the steam bubbleand oil toward the production we ll(s), and recovering produced oil fromthe production well(s). Water may be injected following the gasinjection and steam injection may be repeated.

This invention relates to a process for producing oil from anoil-bearing stratum by steam drive.

Oil is produced from an oil reservoir or stratum by injecting steam thruan injection well penetrating the stratum and driving the steam to oneor more offset production wells. Because of the expense involved insteam injection projects (fuel, water treating, etc.), a number ofinnovations have been utilized in an attempt to reduce the amount ofsteam required in the production process. One such method of decreasingthe steam injection time and quantity of injected steam, aptly calledheat salvage, comprises injecting steam for about of the life of theproduction project and following steam injection with a chasing slug ofwater for the remaining /3 of the production time. In practice, the coldwater is heated by the hot rock in the stratum and this heat stored inthe reservoir by the steam is reclaimed in the hot water, a portion ofwhich may be converted to steam. There still exists the problem ofreducing the steam injection period and the quantity of steam requiredin the production process in order to effect substantial savings in thecost of production of each barrel of oil.

This invention is concerned with a modified steam injection processwhich reduces both the steam injection period and the quantity of steamrequired in any given steam injection project.

Accordingly, it is an object of the invention to provide an improvedsteam injection process for the production of oil from an oil stratum.Another object is to provide a process or method of steam drive thru anoil stratum which reduces the loss and dissipation of heat from theinjected steam. Other objects of the invention will become apparent toone skilled in the art upon consideration of the accompanyingdisclosure.

A broad aspect of the invention comprises injecting a slug of steam intothe stratum from which oil is to be produced, utilizing an injectionwell penetrating the stratum and one or more offset wells as productionwells, and driving the resulting steam bubble thru the stratum towardthe production well(s) with a slug of gas (other than steam), alsoinjected thru the injection well, which displaces oil from the stratumand drives same into the production well(s). The gas injection step ofthe process may then be followed by a water injection step, the waterforcing the gas and the steam slugs thrus the stratum to the productionwell(s).

Gases have a much lower thermal conductivity and specific heat thanwater, which is illustrated in the table below.

wit-matbit.

Patented Feb. 4, 1969 Substance: Heat conductivity Water, 20 C 0.00143Air, 0 C 0.0000568 CO 0 C 0.0000307 CH 78 C 0.0000647 N 78 C 0.0000524The above gases, as well as flue gas, natural gas, and similar gases,are utilized as the gas slug in the process. The use of gas serves atwo-fold purpose. First, because of the lower heat conductivity ofvarious gases, the reservoir rock releases its heat at a slower rate andthus prolongs the collapse of the steam slug (by condensation) for alonger time; second, with the prolonged life of the steam slug, lesssteam is required than when the water is injected directly behind thesteam slug.

From another point of view, gases have a much lower heat capacity orspecific heat than water so that filling the unoccupied pore space witha gas requires only a small fraction of the heat (supplied by the rock)to raise the temperature of the gas to ambient temperature as comparedwith the amount of heat required to raise the temperature of water(under the same circumstances) to ambient temperature. In addition tothe higher specific heat of water, the heat of vaporization of water isalso to be taken into consideration and this is extremely high. Anotherfactor which has an important bearing on the heat absorption of the gasas compared to the water is the vastly smaller mass of gas (based on itsweight) in the available pore space as compared to water. Thus,injection of a suitable gas directly behind the steam slug is moreeffective in pushing the steam bubble and heated zone thru the stratumthan water is.

In a steam driveat high pressure, it is advantageous to utilize naturalgas which is available at the locus of the project at high pressure,thereby minimizing the compression costs. However, if compression isnecessary, the heat of compression is in the compressed gas and thisheat is effective when the gas in injected into the stratum to chase thesteam. Compression of other gases also raises the temperature of thecompressed gas and therefore the compression cost is not lost since mostof the heat is imparted to the reservoir or stratum.

Carbon dioxide is particularly effective as a chasing gas in view of itsbeneficial effect on reservoir liquids. Liquid CO at pressure greaterthan the critical pressure (1073 p.s.i.a.) and below the criticaltemperature (88 F.) is preferably injected behind the steam slug when COis to be utilized. As the injected liquid CO is heated to reservoirtemperature, it vaporizes, expands, and fills the reservoir spacepreviously occupied by steam. This produces a beneficial effect on theproduction of oil.

The most simple embodiment of the invention omits the water injectionstep and utilizes the injected gas solely to drive the steam slug andheat zone thru the stratum toward the production well(s). In thisoperation, the steam generators are shut down after a considerableperiod of steam injection to provide a steam bubble occupying asubstantial section of the stratum within the production pattern and gasis injected so as to drive the steam bubble thru the reservoir into theproduction wells. It is also feasible to inject a sizable slug of gasand then inject a second steam slug to form a second steam bubble andfollow this second steam injection with gas injection.

The injection of gas in accordance with the instant invention tends todrive the injected steam and heated zone farther into the productive oilzone so as to avoid steam condensation around the injection well,resulting in the reduction of pressure at this point and causing thesteam bubble to be pulled back toward the injection well.

Air is an excellent gas for the gas injection step following steaminjection. Air is the cheapest and most readily available gas and it iseffective in driving the steam bubble and heat zone thru the reservoir.In some cases the injection of air into the hot stratum causes oxidationof the residual in-place oil behind the steam bubble. The steam driveoften leaves 8 to 10 percent of the oil as residue and hot air incontact with this residual oil causes oxidation thereof with productionof additional heat and expansion of the gas slug by formation ofcombustion gases and expansion due to the heating provided by thecombustion period. This in situ combustion drive behind the steam bubbleaids materially in the displacing of oil from the stratum.

The steam drive process of the invention is effectively applied to astrat-um which is hot from a previous in situ combustion drive and whichleaves substantial concentrations of oil in the reservoir. By injectingeither steam or water into the hot stratum a steam bubble is establishedwhich can readily be driven thru the stratum as described above, usinggas drive alone or gas drive behind the steam, followed by water driveof the gas.

The advantages of the invention are in the conservation of heat in thereservoir or stratum because of the low dissipation of heat from therock to the gas as contrasted to a substantially greater dissipation ofheat from the rock to water when a water slug is injected immediatelybehind a steam slug. Further, it is possible to use a smaller steam slugbecause of the considerably longer cooling-off period (of the reservoir)than when water is injected directly behind the steam slug.

Certain gases offer other advantages. CO for example, swells theresidual oil and is miscible with the forward steam slug and therearward water sl-ug. Methane or natural gas also tends to swell theresidual oil displaced by the steam.

The amount of the steam slug to be injected is in the range of 0.5 toseveral pore volumes of the sweep pattern and the temperature of thesteam is in the range of about 300 to 750 F. The size of the injectedsteam slug for multiple injections, each followed by gas slugs, is inthe lower portion of the range. The injected gas slug may be smallerthan the steam slug because of the non-condensing nature of the gas. Agas slug in the range of 0.1 to 1 or more pore volumes may be utilized.

Certain modifications of the invention will become apparent to thoseskilled in the art and the illustrative details disclosed are not to beconstrued as imposing unnecessary limitations on the invention.

I claim:

1. A steam drive process for producing oil from a permeable oil stratumpenetrated by an injection well and as least one ofiset production well,which process comprises, in combination, the steps of:

(a) first depositing steam as the principal driving medium in saidstratum around said injection well for an extended period of time toform a steam bubble occupying a section of said stratum in the wellpattern;

(b) then, injecting a following slug solely of gas other than steamthrough said injection well directly behind said steam so as to drivesaid steam bubble and condensate therefrom deeper into said stratumtoward said production well to displace oil into same;

(c) then, injecting a second substantial slug of steam directly behindsaid gas slug of step (b);

((1) then, injecting a second slug of gas other than steam directlybehind said steam of step (c); and

(e) producing oil from said production well.

2. The process of claim 1 wherein said slug of gas is combustion gas.

3. The process of claim 1 wherein said slug of gas is C0 4. The processof claim 1 wherein said slug of gas comprises natural gas components.

5. The process of claim 1 wherein said slug of gas is arr.

6. The process of claim 1 wherein said slug of gas is N 7. The processof claim 1 wherein said stratum is initially at an elevated temperaturefollowing an in situ combustion drive and the steam of step (a), atleast, is formed in situ by injecting water into the hot stratum throughsaid injection well.

References Cited UNITED STATES PATENTS 2,584,606 2/ 1952 Merriam et al16611 2,813,583 11/1957 Marx et al. 16640 X 2,897,894 8/ 1959 Draper eta1 166-9 3,036,632 5/1962 Koch et a1. 166-11 3,042,114 7/1962 Willman16640 X 3,294,167 12/1966 Vogel 166-11 FOREIGN PATENTS 511,768 8/1939Great Britain.

STEPHEN I. NOVOSAD, Primary Examiner.

US. Cl. X.R. 166-273, 305

